Current non-powered tools for tissue dissection such as the scalpel and associated instruments tend to cause bleeding at the incised area. There is nothing inherent in the tool which achieves hemostasis. Electrocautery, or more broadly, radio frequency electromagnetic energy based tools, can achieve hemostasis through the deposition of electrical currents in the tissue which effectively cauterize the bleeding tissue through resistive heating. The disadvantage of the electrocautery tools is that they do not make good fine-dissection tools, since the energy deposition is not precisely controlled, and the tool tips can stick to the tissue.
The use of ultrasonic energy in surgical procedures is known to those skilled in the art to be a valuable resource for cutting and fragmenting tissue of a patient. Most of these apparatus incorporate a sinusoidal driving signal which causes the mechanical tip to vibrate at a selected frequency, usually in the range of 20 kHz. to 60 kHz. The sinusoidal drive signal may be derived from a square wave at the same frequency or it may be inherent in the design. Further, some devices utilize duty-cycle modulated waveforms to achieve a desired effect.
It is also known to those skilled in the art that at the lower end of the preferred frequency spectrum, i.e. 20 kHz. to 40 kHz., larger tip displacements are possible. It is also known that larger tip displacements provide a better tissue cutting effect than small tip displacements.
Ultrasonic energy at the high end of the preferred frequency spectrum, i.e. 40 kHz. to 60 kHz., can have more hemostatic effect. This is due to the increased absorbance of higher frequency energy by tissue. However, larger tip displacements are not feasible at these frequencies. Therefore, devices which operate in this realm have reduced tissue cutting performance.
Several ultrasonic surgical devices have been disclosed which attempt to provide both tissue cutting and coagulation functions, U.S. Pat. No. 5,026,387 assigned to Ultracision, Inc., has several operating modes. Power is transmitted at an ultrasonic frequency to the instrument at a relatively high level when a surgical function is to be effected and the load on the instrument is relatively high. Power is transmitted to the instrument at a relatively low level during idle periods of use. A high power coagulation mode is manually selectable with automatic return to an idle power level when the blade is not in contact with tissue. There is no claim to operating at different frequencies.
In U.S. Pat. No. 5,263,957 assigned to Ultracision, Inc., the shape of an ultrasonic blade is disposed to provide a cutting function and a coagulation function. One portion of the blade is disposed to cut tissue. A second portion of the blade is disposed to provide frictional coupling of the blade edge and tissue to obtain tissue motion with resulting generation of heat and a hemostatic effect.
Foreign Patent WO 9,314,709 discloses an ultrasonic blade with a hook formed in the blade having flat, non-sharpened, relatively dull edges. This facilitates the cutting and coagulation of the tissue upon application of the ultrasonic energy to the tissue.